Cathode ray tube



e. F. BR'ETT 2,211,844

Aug. 20, 1940.

CATHODE RAY TUBE v Filed Jan. '7, 1937 =l l i INVENTOR GEORGE FAIRBURN BRETT ATTO R N EY Patented Aug. 20, 1940 CATHODE RAY TUBE George Fairburn Brett, London, England, assigng or to Radio Corporation of America, a corporation of Delaware Application January 7, 1937, Serial No. 119,474 In Great Britain February 24, 1936 2 Claims.

This invention relates to cathode ray tubes.

A small cheap cathode ray tube capable of operating with a low anode voltage (for example about 250 volts) and having a small length of cathode ray beam and a metal backed fluorescent screen would be of advantage for many practical purposes, for example in radio direction finding equipment and in other cases where compactness and cheapness of replacement of the tube are important. The principal object of the present invention is to provide an improved cathode ray tube satisfying these requirements. From the point of view of cheapness and compactness moreover it is desirable that the envelope of the tube be of simple cylindrical shape rather than of the somewhat elaborate part-conical shape common with present day cathode ray tube oscillographs such as are used for television reception purposes. Accordingly in carrying out the present invention the envelope of a cathode ray tube is preferably, though not necessarily, made 'of simple cylindrical shape and, as will be seen later, the invention is such that the use of such a simple cylindrical envelope is facilitated.

According to this invention a cathode ray tube of the kind including an electron gun system and a co-operating screen structure is characterized in that the said screen structure is mounted transversely with respect to the gun axis and is off-set from the gun axis, and means are provided for deflecting the cathode ray so that it falls upon the said transverse off-set screen structure. Preferably the screen has its surface parallel to the gun axis. In practice the screen structure will generally be constituted by a metal plate having a layer of fluorescent material thereon.

The invention finds a particular practical application in cathode ray tubes having a gun system such as described in the specification accompanying my co-pending application No. 120,680, filed January 15, 1937 though, of course, it is not limited, in its application, thereto.

The invention is illustrated in the accompanying drawing which shows a preferred way of carrying it into effect. In this embodiment of the invention a cathode ray tube comprises a cylindrical envelope E having a re-entrant footstep of usual construction at one end. Carried, from this footstep is an electron gun G comprising the usual electrode system the axis of the gun corresponding with the axis of the cylindrical envelope. Also mounted within the cylindrical envelope and preferably carried by one or more supporting rods R fused through the end of the envelope remote from the footstep, is a screen structure S consisting of a flat metal plate with a layer of fluorescent material thereon. This screen is mounted in such a position that its surface is parallel to the axis of the envelope and is a short distance therefrom so that the said screen is transverse and off-set with respect to the electron gun axis. The mid-point of the screen surface may be about 4 from the electron gun for a tube adapted to operate with an anode voltage somewhere between 150 volts and 350 volts or thereabouts. It will be appreciated that with a tube constructed as so far described, if the usual deflecting voltages were applied to deflect the cathode ray, the said ray would not trace a pattern or picture upon the screen for it would (at least over a portion of its traverse) miss the screen altogether and this difliculty is avoided in carrying out this invention by providing a fixed component of supplementary deflecting field so as to bend the cathode ray towards the screen surface in the manner shown in the drawing.

, The normal usual ray deflecting fields may be superimposed upon this fixed component of ray deflecting field so that the ray may trace a picture or pattern upon the transverse screen.

Preferably the necessary ray deflection is obtained entirely electro-magnetically by means of two mutually perpendicular pairs of ray deflecting coils in manner known per se, one pair producing deflection in the direction of the axis of :1

the tube, see for instance positions a and b, and serving also to provide the fixed component of ray deflection. This pair of coils could conveniently be the recipient of that ray deflecting wave which provides the time, base. The other pair of coils produces deflection in the perpendicular direction. It will be appreciated that if the varying deflecting forces applied were such as would produce a rectangular scanned area upon a screen perpendicular to the gun axis the shape of the scanned area actually produced (by the same deflecting forces) upon the transverse off-set screen will be not a, rectangle but a trapezium. For many purposes, however, this will not matter, but if in any case much distortion of scanned area is objectionable, it may be corrected for as known per se, e. g. by suitably modifying the deflecting wave forms applied for scanning action.

Although, as above stated, electro-magnetic deflection is preferably employed, this is not a necessary feature of the invention for electrostatic deflection may be used. For example electrostatic deflection may be employed to provide mutually perpendicular components of scanning action, and a magnetic field of fixed value may be employed to provide the fixed component of deflection necessary to cause the ray to fall upon the transverse off-set screen.

It will be appreciated that the pattern or picture traced upon a screen in a tube in accordance with this invention may be very easily seen since the screen area is parallel or nearly parallel to the tube axis, and the said picture may accordingly be viewed directly through the side wall of the tube. It is also simple matter to place a viewing lens system L externally-of the tube and With its axis perpendicular to the surface of the screen.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed, I declare that what I claim is:

1. In a cathode ray system, a cathode ray tube having an electron gun for producing a cathode ray beam, a luminescent target element substantially parallel to the axis of said gun and parallel to the longitudinal axis of said tube, means for controlling the deflection of said beam to cause it to scan the target lei-directionally, and means for initially biasing said deflecting means electrically to alter the normal beam projection path in one direction so that the initial position of said beam is such that said beam impinges upon said luminescent target element.

2. In a, cathode ray system, a cathode ray tube having an electron gun for producing a cathode ray beam, a luminescent target element positioned substantially parallel to the axis of said gun and to the longitudinal axis of said tube, electromagnetic means for controlling the deflection of said beam to cause it to scan the target lei-directionally, and means for initially biasing said electromagnetic deflecting means to alter the normal beam deflection path so that the initial position of said beam is such that said beam impinges upon said luminescent target element.

GEORGE FAIRBURN BRETT. 

